Diabase

Diabase (/ˈdaɪ.əˌbeɪs/), also called dolerite (/ˈdɒl.əˌraɪt/) or microgabbro, is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine-grained to aphanitic chilled margins which may contain tachylite (dark mafic glass).

Diabase is the preferred name in North America, while dolerite is the preferred name in the rest of the English-speaking world, where sometimes the name diabase refers to altered dolerites and basalts. Some geologists prefer to avoid confusion by using the name microgabbro.

The name diabase comes from the French diabase, and ultimately from the Greek διάβασις diábasis 'act of crossing over, transition', whereas the name dolerite comes from the French dolérite, from the Greek δολερός dolerós 'deceitful, deceptive', because it was easily confused with diorite.

Diabase normally has a fine but visible texture of euhedral lath-shaped plagioclase crystals (62%) set in a finer matrix of clinopyroxene, typically augite (20–29%), with minor olivine (3% up to 12% in olivine diabase), magnetite (2%), and ilmenite (2%). Accessory and alteration minerals include hornblende, biotite, apatite, pyrrhotite, chalcopyrite, serpentine, chlorite, and calcite. The texture is termed diabasic and is typical of diabases. This diabasic texture is also termed interstitial. The feldspar is high in anorthite (as opposed to albite), the calcium endmember of the plagioclase anorthite-albite solid solution series, most commonly labradorite.

Diabase is usually found in smaller, relatively shallow intrusive bodies such as dikes and sills. Diabase dikes occur in regions of crustal extension and often occur in dike swarms of hundreds of individual dikes or sills radiating from a single volcanic center.

The Palisades Sill which makes up the New Jersey Palisades on the Hudson River, near New York City, New York, United States, is an example of a diabase sill. The dike complexes of the British Tertiary Volcanic Province includes Skye, Rum, Mull, and Arran of western Scotland, the Slieve Gullion region of Ireland, and dolerite dike swarms extending across northern England towards the Midlands, for example Rowley Rag. Parts of the Deccan Traps of India, formed at the end of the Cretaceous, also include dolerite. It is also abundant in large parts of Curaçao, an island off the coast of Venezuela. Another example of diabase dikes has been recognized in the Mongo area within the Guéra Massif of Chad in Central Africa.

In the Death Valley region of California, Precambrian diabase intrusions metamorphosed pre-existing dolomite into economically important talc deposits.

In the Thuringian-Franconian-Vogtland Slate Mountains of central Germany the diabase is entirely of Devonian age. They form typical domed landscapes, especially in the Vogtland. One geotourist attraction is the Steinerne Rose near Saalburg, a natural monument, whose present shape is due to the typical weathering of lava pillows.